Schottky Barrier diode with controlled characteristics

ABSTRACT

A self-isolated Schottky Barrier diode structure and method of fabrication are disclosed for generating a device having controlled characteristics. An opening is made through an oxide layer over a central region of an n-type semiconductor substrate. The opening has inclined sidewalls over an annular region surrounding the central region of the substrate. An n-type dopant layer is ion implanted through the opening and the surrounding oxide layer. This controls the barrier height for the Schottky Barrier diode and controls the lifetime of minority carriers in the outside region of the substrate. This has the effect of minimizing PNP parasitic transistor action. A Schottky Barrier contact is formed in the opening through an oxide layer creating a rectifying junction with the semiconductor substrate in the central region.

This is a continuation, of application Ser. No. 971,166 filed Dec. 20,1978, now abandoned.

FIELD OF THE INVENTION

The invention disclosed broadly relates to semiconductor devices andmore particularly relates to improvement in diode devices.

BACKGROUND OF THE INVENTION

In the implementation of high speed logic, it is often desirable to usea Schottky Barrier diode to prevent the saturation of the switchingtransistor. The degree of improvement is limited however by the nominalbarrier height, the variability in barrier height due to the epitaxialdoping level, and the series resistance between the Schottky Barrierdiode and the collector contact.

Typical prior art formation of Zener diodes on a large scale integratedcircuit chip required extra processing steps to achieve a controlled lowbreakdown voltage comparable to the signal levels on the chip.

In typical prior art applications of Schottky Barrier diodes in largescale integrated circuitry, the Schottky Barrier contact is made at onepoint on the surface of the semiconductor substrate and serves as theanode for the device. The Schottky Barrier diode contact may be aluminumwhich is placed directly in contact with an n-type epitaxial layer ofsilicon. Located at a position proximate to the Schottky Barrier diodecontact is an ohmic contact which serves as the cathode of the diode andis formed by placing an aluminum layer in contact with an n+ diffusionin the epitaxial layer. When the anode is biased positively with respectto the cathode, conventional current flows from the Schottky Barriercontact, through the bulk of the epitaxial layer to the negativelybiased ohmic contact. A voltage drop associated with the current flowoccurs across the Schottky Barrier which is a function of the epitaxialdoping and the bulk epitaxy resistance. Thus, the forward biased diodevoltage drop for a Schottky Barrier diode is the sum of the potentialdrop across the junction V_(j) plus the series resistance voltage dropthrough the epitaxial layer between the anode and the cathode. Typicaln-type epitaxial silicon layers have a conductivity which varies by asmuch as plus or minus 50%. Thus the contribution of the junction andseries resistance voltage drop across the Schottky Barrier diode canvary significantly from one LSI process batch to another. Thus SchottkyBarrier diodes have had only limited usage in large scale integratedcircuit design in the prior art due to their substantial variability intheir resulting characteristics.

OBJECTS OF THE INVENTION

It is therefore an object of the invention to provide an improvedSchottky Barrier diode.

It is still another object of the invention to provide controlledcharacteristics for Schottky Barrier diode.

It is still a further object of the invention to lower the barrier of aSchottky diode in an improved manner.

It is yet a further object of the invention to reduce the seriesresistance of the Schottky Barrier diode.

It is yet another object of the invention to provide a tightermanufacturing tolerance for the barrier height and series resistance ofa Schottky Barrier diode.

It is yet a further object of the invention to provide a reducedparasitic action between Schottky Barrier diodes and adjacent p-typediffusions or other Schottky Barrier diodes.

It is still a further object of the invention to provide a lowresistance region self-aligned to a Schottky Barrier diode.

It is yet further object of the invention to provide an improvedSchottky Barrier Zener diode.

It is yet a further object of the invention to provide an improvedSchottky Barrier Zener diode with controlled characteristics.

SUMMARY OF THE INVENTION

These and other objects, features and advantages of the invention areaccomplished by the Schottky Barrier diode with controlledcharacteristics disclosed herein. A self-isolated Schottky Barrier diodestructure and method of fabrication are disclosed for generating adevice having controlled characteristics. An opening is made through anoxide layer over a central region of an n-type semiconductor substrate.The opening has inclined sidewalls over an annular region surroundingthe central region of the substrate. An n-type dopant layer is ionimplanted through the opening and the surrounding oxide layer. Theion-implanted layer has a substantially Gaussian distribution of itsconcentration with respect to depth in the substrate. In the centralregion beneath the opening, the end of the distribution intersects thesurface of the substrate at a second concentration which is between 2and 10 times greater than the background concentration of the substrate.This controls the barrier height for the Schottky Barrier diode. Thedistribution has a peak concentration which is located beneath thesurface of the substrate at a first distance in the central region. Thispeak concentration gradually rises toward the substrate surfacesubstantially parallel with the inclined sidewalls of the opening in theannular region of the substrate. In the region of the substratesurrounding the annular region, where the ion implantation takes placethrough the full thickness of the oxide, the distribution intersects thesubstrate surface at a third concentration which is at least 10 timesgreater than the second concentration where the end of the distributionintersects the surface in the central region. This third concentrationcontrols the lifetime of minority carriers in the outside region of thesubstrate. This has the effect of minimizing PNP parasitic transistoraction. A Schottky Barrier contact is formed in the opening through theoxide layer creating a rectifying junction with the semiconductorsubstrate in the central region. The structure can be extended toinclude a second opening through the oxide layer over the outside regionof the substrate and the formation of an ohmic contact in the secondopening to make a low series resistance connection to the SchottkyBarrier diode. The resulting device achieves a lower barrier for theSchottky Barrier diode and a lower series resistance to the SchottkyBarrier diode than was available in the prior art. In addition, the ionimplant can be used to form a Schottky Barrier Zener diode withcontrollable characteristics.

DESCRIPTION OF THE FIGURES

These and other objects, features and advantages of the invention willbe more fully appreciated with reference to the accompanying figures.

FIG. 1 is a cross-sectional view of a first stage in the fabrication ofthe Schottky Barrier diode with controlled characteristics whereopenings are formed through a silicon dioxide layer for the anode andcathode of the diode in a silicon semiconductor substrate.

FIG. 2a is a cross-sectional view of a later stage in the fabrication ofthe Schottky Barrier diode with controlled characteristics where aphotoresist-defined n-type buried layer has been ion implanted into thesemiconductor substrate.

FIG. 2b shows the completion of the device shown in FIG. 2a, with thedeposition of a metal layer.

FIG. 3 is a doping profile through section A--A' of FIG. 2.

FIG. 4 is a doping profile through the cross-section B--B' of FIG. 2.

FIG. 5 is a graph of the current versus voltage characteristics for theforward and the reverse operation of the Schottky Barrier diode withcontrolled characteristics.

DISCUSSION OF THE PREFERRED EMBODIMENT

A self-isolated Schottky Barrier diode structure and method offabrication are disclosed for generating a device having controlledcharacteristics. An opening is made through an oxide layer over acentral region of an n-type semiconductor substrate. The opening hasinclined sidewalls over an annular region surrounding the central regionof the substrate. An n-type dopant layer is ion implanted through theopening and the surrounding oxide layer. The ion-implanted layer has asubstantially Gaussian distribution of its concentration with respect todepth in the substrate. In the central region beneath the opening, theend of the distribution intersects the surface of the substrate at asecond concentration which is between 2 and 10 times greater than thebackground concentration of the substrate. This controls the barrierheight for the Schottky Barrier diode. The distribution has a peakconcentration which is located beneath the surface of the substrate at afirst distance in the central region. This peak concentration graduallyrises toward the substrate surface substantially parallel with theinclined sidewalls of the opening in the annular region of thesubstrate. In the region of the substrate surrounding the annularregion, where the ion implantation takes place through the fullthickness of the oxide, the distribution intersects the substratesurface at a third concentration which is at least 10 times greater thanthe second concentration where the end of the distribution intersectsthe surface in the central region. This third concentration controls thelifetime of minority carriers in the outside region of the substrate.This has the effect of minimizing PNP parasitic transistor action. ASchottky Barrier contact is formed in the opening through the oxidelayer creating a rectifying junction with the semiconductor substrate inthe central region. The structure can be extended to include a secondopening through the oxide layer over the outside region of the substrateand the formation of an ohmic contact in the second opening to make alow series resistance connection to the Schottky Barrier diode. Theresulting device achieves a lower barrier for the Schottky Barrier diodeand a lower series resistance than was available in the prior art. Inaddition, the ion implant can be used to form a Schottky Barrier Zenerdiode with controllable characteristics.

FIG. 1 is a cross-sectional view of a first stage in the manufacture ofthe Schottky Barrier diode with controlled characteristics. A p-typesilicon substrate 2 has deposited on its surface an n- epitaxial siliconlayer 4 to a thickness of approximately 2 microns and a conductivity ofapproximately 2 ohm centimeters. A 3000 A thick layer of silicon dioxide6 is grown on the surface of the epitaxial layer 4 and a window 8 openedwhere the ultimate location of the cathode for the Schottky Barrierdiode will be. An n+ diffusion 10 is then deposited through the window 8to form the ohmic contact for the cathode in the ultimate device. Thisis followed by the opening of the window 12 through the silicon dioxidelayer at the position where the anode for the ultimate device will belocated, proximate to the location of the window 8, having ananode-to-cathode separation distance of approximately 5 microns.

FIG. 2A shows a later stage in the process for fabricating the SchottkyBarrier diode with controlled characteristics. The windows 8 and 12 havebeen formed by an etching process whereby a tapered contour for thesidewalls 9 and 13 is formed which is less than or approximately equalto 45 degrees as typically achieved with prior art etching techniques.

As is shown in FIG. 2a, a 2 micron thick layer of photoresist 14 isdeposited on the surface of the silicon dioxide layer 6 leaving thewindow 15 exposed which encompasses both the anode and the cathoderegion of the Schottky Barrier diode to be formed. The photoresist layer14 will serve as an ion-implantation mask preventing the penetration ofaccelerated ions into the epitaxial layer 4. A 300 Kev energyphosphorous ion implant is directed into the window region 15 shown inFIG. 2a at a dosage of approximately 1×10¹⁴ atoms per square centimeter.A subsequent anneal or drive-in is done at approximately 900°-1000° C.for 30 minutes to activate the implanted carriers and drive them to adeeper junction if desired. The energy of approximately 300 Kev forphosphorous ions is just sufficient to allow the ions to penetratethrough the 3000 A thickness of silicon dioxide 6 at locations 17, 19and 21. The peak of the Gaussian distribution after annealing for theion-implanted profile, as is shown in FIG. 4, falls at or near theinterface between the silicon dioxide layer 6 and the silicon epitaxiallayer 4 at the locations 17, 19 and 21. Since there is no silicondioxide covering the epitaxial layer in the window regions 8 and 12, thephosphorous implant will follow the contour of the oxide step at thesidewalls 9 and 13 maintaining a lateral as well as vertical spacingfrom the metal in the contact region. The profile of the concentrationof the ion-implanted region at the section A--A' in the window 12 isshown in FIG. 3. It may be seen there that the tail of the distribution23 intersects the upper surface of the epitaxial layer 4 at a selectedconcentration which is higher than the doping layer of the epitaxiallayer 4 and serves to predictably adjust the barrier height for theresulting Schottky Barrier diode formed in the window 12.

The relative distance of the peak of the implant concentration 25 to theSi surface controls the reverse bias depletion layer spreading and hencethe reverse bias breakdown.

The integral of the implanted dopant determines the series resistancefrom the anode to the cathode. In addition the integral of the dopantalso determines the recombination rate of minority carriers injected bythe Schottky Barrier diode, the higher the integrated doping, the fasterthe minority carrier recombination rate. As a result, the parasitic PNPaction due to minority carrier injection between adjacent SchottkyBarrier diodes can be minimized.

The resulting ion-implanted layer 24 in FIG. 2a represents the peak ofthe Gaussian distribution for the ion implantation plus or minus onestandard deviation, as may be seen in FIGS. 3 and 4.

FIG. 2b shows the completion of the process for fabricating the SchottkyBarrier diode with controlled characteristics where an aluminum layer isdeposited forming the contact 26 and contact 28 in the windows 12 and 8,respectively. The aluminum contact 26 forms a Schottky Barrier to the nepitaxial layer 4 in the window 12, forming the anode of the SchottkyBarrier diode device. The aluminum contact 28 in the window 8 directlycontacts the n+ diffusion 10, thereby forming an ohmic contact which isthe cathode for the Schottky Barrier diode device. The ion-implantedregion 24 lowers the barrier height of the Schottky Barrier diode in thewindow 12 by providing a controlled higher doping level at the interfaceas is shown in FIG. 3. In addition, the ion-implanted layer 24 providesa reduced series resistance between the anode at the window 12 and thecathode at the window 8 since current flowing from the anode to thecathode need not flow through the higher resistance bulk region 4 of theepitaxial layer but may flow through the lower resistance ion-implantedregion 24. Furthermore, the resulting characteristics of the SchottkyBarrier diode device have a tighter manufacturing tolerance in both thebarrier height for the diode and the series resistance for the diodebecause of the greater control over the dopant concentration which isavailable for the ion-implanted region, with respect to that of theepitaxial layer 4. Still further, under certain conditions, the SchottkyBarrier at the window 12 will generate holes into the n-type epitaxiallayer 4. These holes can diffuse away from the anode region in prior artstructures leading to undesired parasitic action between adjacentSchottky Barrier diodes or adjacent p diffusions. This is minimized inthe structure shown in FIG. 2b by virtue of the higher re-combinationrate provided by the n-type doped ion-implanted region 24.

Reference to FIG. 5 will illustrate the operating characteristics of theSchottky Barrier diode with controlled characteristics. The forwarddiode voltage drop is substantially reduced over that available in theprior art Schottky Barrier diodes because of the reduced seriesresistance and lower barrier height.

The series resistance between the anode and cathode of the SchottkyBarrier diode is reduced by the implanted region 24 (FIG. 2a). Since theresistance is a function of the bulk doping levels between the anode andcathode, the implant serves to increase the integrated doping level, tolower the series resistance in the diode. The increase in the integrateddoping surrounding the diode also serves to decrease minority carrierlifetime, and hence any parasitic action to adjacent devices.

The forward voltage drop across the diode is selectively adjusted by theintersection of the tail of the implant at the metal-Si interface. Thebarrier height at the metal-Si interface is adjusted by the SchottkyBarrier lowering due to the adjustment of the intrinsic electric fieldby the tail 23 of the implanted region 24 at the interface.

In addition to the forward bias diode characteristics of the SchottkyBarrier diode shown in FIG. 2b, improved reverse breakdowncharacteristics are also achieved with the device. In particular, anadjustable Zener diode is formed by the structure. Reference to FIG. 3will illustrate how the improved and controlled Zener reverse bias diodecharacteristics are obtained. If there were no ion-implanted region 24in the structure shown in FIG. 2b, when the Schottky Barrier diodeformed in window 12 was reverse biased, a depletion layer surroundingthe junction of the diode would spread into the bulk of the epitaxiallayer 4. Contrast this with the situation shown in FIG. 2b where then-type ion-implanted region 24 has raised the n-type dopantconcentration by three orders of magnitude at a depth of approximately0.4 microns below the surface of the epitaxial layer. In this situation,the spreading of the depletion layer surrounding the junction of theSchottky Barrier diode is prevented by the ion-implanted region 24,thereby increasing the peak electric field for a given voltage. As aresult, the diode approaches the avalanche breakdown at a lower voltagethan would otherwise be the case. Thus, proper selection of the implantenergy which locates the peak of the Gaussian distribution for theion-implanted region as shown in FIG. 3, determines the resultingreverse biased avalanche breakdown voltage for the Zener diode. Thus, byadjusting the acceleration voltage for the ion-implanted region 24, andin addition by adjusting the dose or concentration of the dopant in theion-implanted region 24, the Zener breakdown voltage for the resultingSchottky Barrier diode can be controlled. A substantial advantageaccrues to this structure as shown in FIG. 5 since the Zener breakdownvoltage can be made substantially smaller than the typical prior artvoltage so that it can approximate signal voltage levels on the largescale integrated circuit chip without unduly complex manufacturingtechniques.

It is important to note that the fabrication of Schottky Barrier diodeswith this technique can be designed to achieve several specific devicecharacteristics. The specific choices of implanted dose, energy, annealtime and temperature, and oxide thickness 19, determine the surfaceconcentration and hence barrier height, the integrated doping levels andhence series resistance and minority carrier lifetime, and the distancefrom the metal interface to the peak of the implanted region and hencethe depletion layer spreading and resulting breakdown. As can been seen,the single process step of fabricating the implanted region 24 allowsfor the design of Schottky Barrier diodes with specific devicecharacteristics, where the specific characteristics are a function ofthe implant energy, dose, and oxide thickness.

In addition it is important to note that the implanted region 24 followsthe contour of the surface oxide region 19 and as a result the dopingprofile peak is self-aligned to the Schottky Barrier diode contactopening 12. This results in improved device density and performance.

While the invention has been particularly shown and described withrespect to the preferred embodiments thereof, it will be understood bythose skilled in the art that the foregoing and other changes in formand details may be made therein without departing from the spirit andthe scope of the invention.

Having thus described our invention, what we claim as new, and desire tosecure by Letters Patent is:
 1. A self-isolated Schottky Barrier diodewith controlled characteristics, comprising:a semiconductor substrate ofa first conductivity type and first concentration; an insulating layeroverlying said substrate having a first thickness; a first openingthrough said insulating layer over a central region, having inclinedsidewalls over an annular region surrounding a first region in saidsubstrate; an ion-implanted layer of said first conductivity type insaid central region having a substantially Gaussian distribution of itsconcentration with respect to depth of implantation in said substrate,with the end of said distribution intersecting the surface of saidsubstrate at a second concentration of between 2 and 10 times said firstconcentration, and having a peak concentration at a first distance insaid first region, said second concentration controlling the barrierheight for the Schottky Barrier diode; said ion-implanted layergradually rising toward said substrate surface and substantiallyparallel with said inclined sidewall; said first distance being greaterthan said first thickness for said insulating layer so that saiddistribution intersects said substrate surface at an outside regionoutside of said annular region at a third concentration which is atleast 10 times greater than said second concentration, said thirdconcentration controlling the lifetime of minority carriers in saidoutside region; a Schottky Barrier contact formed in said first openingforming a rectifying junction with said semiconductor substrate in saidcentral region; a second opening through said insulating layer over saidoutside region of said substrate; an ohmic contact formed in said secondopening, making electrical contact with said ion-implanted layer in saidoutside region; whereby a self-isolated Schottky Barrier diode is formedwith controlled characteristics and a lower series resistance connectionis made to said Schottky Barrier diode.
 2. The structure of claim 1,wherein said sidewall inclination is less than 45 degrees with respectto said substrate surface.
 3. The structure of claim 1 wherein saidthird concentration is greater than 10¹⁸ dopant atoms per cubiccentimeter.
 4. The structure of claim 1, wherein said Schottky Barriercontact is composed of a material selected from the group consisting ofan aluminum silicon alloy, platinum, tantalum, chromium, molybdenum, andtitanium tungsten alloy.
 5. The structure of claim 1, wherein saidSchottky Barrier diode is the collector of a bipolar transistor.
 6. Thestructure of claim 1, wherein said first conductivity type is n type. 7.A self-isolated Schottky Barrier Zener diode with controlledcharacteristics, comprising:a semiconductor substrate of a firstconductivity type and first concentration; a Schottky Barrier contactformed in said first opening forming a rectifying junction with saidsemiconductor substrate in said central region; a second opening throughsaid insulating layer over said outside region of said substrate; anohmic contact formed in said second opening, making electrical contactwith said ion-implanted layer in said outside region; whereby aself-isolated Schottky Barrier Zener diode is formed with controlledcharacteristics and a lower series resistance connection is made to saidSchottky Barrier Zener diode.